1. Field of the Invention
The present invention relates to an unit cell grouping method and a battery pack configuration and, more particularly, to a method for optimizing selection of unit cells to make a battery pack so as to secure good performance of the battery pack with primary or secondary batteries connected to one another in series, in parallel or in combination of them.
2. Related Prior Art
As is well known in the art, in order to secure good performance of a battery pack, it is desirable to select unit cells of a similar performance characteristic in the manufacture of the battery pack with unit cells connected to one another in series, in parallel, or in combination of them. However, the general method of evaluating a battery pack based on the initial capacity is unsatisfactory for securing the similarity for the unit cells of the battery pack, because the unit cells of the same capacity may be different from one another in aging speed and internal characteristic, such as resistance. A battery pack, if configured with parallel unit cells of a different internal characteristic, may encounter an overload on the unit cell having a smaller resistance during a discharge. This means that a current higher than a predetermined current value is flowing to the cell to rapidly shorten the battery life. Namely, in the battery pack with parallel unit cells of a different internal characteristic, the unit cells are equally deteriorated in performance due to the unit cell having a higher resistance. In case of a battery pack with serial unit cells of a different internal characteristic, the cell of a higher resistance has a higher voltage and causes rapid aging due to a voltage difference from the other cells. Such a rise of voltage keeps going and, without a separate protection circuit, may result in explosion of the battery pack.
Until now, the measurement of impedance has been adapted to sort out defective unit cells. For example, the measurement of impedance at a fixed frequency for a short time of less than one second as disclosed in U.S. Pat. No. 3,873,911 can be employed for detection of unit cells having a lower capacity than the average value. But the measurement of impedance at a fixed high frequency cannot always predict the accurate capacity of the unit cells. This is because the individual unit cells have a different tendency to increase the impedance in the low frequency region, which means that the unit cell of high impedance at 1 kHz may exhibit a low impedance value in the low frequency region.
The method of selecting unit cells to make a battery pack based on the measurement of open circuit voltage may also present erroneous results, because the open circuit voltage does not represent the accurate internal characteristic of the unit cells and varies insensitively according to the discharge state of the unit cells.
For quality control of the batteries to sort out defective batteries, many methods for analyzing the impedance spectrum of batteries have been proposed. These methods are based on the measurement of dynamic resistance at a fixed frequency (U.S. Pat. No. 3,873,911); variations of voltage and capacity during a discharge (European Patent No. 0119547); response signals fed back from a battery upon applying a charging signal (U.S. Pat. No. 3,808,487); difference of phase between AC voltage across the battery and the AC current applied to the battery (U.S. Pat. No. 3,984,762); the measurement of internal resistance of lithium-iodine batteries (U.S. Pat. No. 4,259,639); difference of argument between impedance values measured at two different frequencies (U.S. Pat. No. 4,743,855); the measurement of angle built by straight line between impedance point at several frequencies in Real vs. Imaginary part plot (Nyquist plot) (U.S. Pat. No. 5,241,275); and the analysis of parameters obtained by fitting impedance spectrum to an equivalent circuit model (U.S. Pat. No. 6,208,147).
These conventional methods have the same problem in that only a part of the information regarding the internal characteristics of the battery obtained from the measurement can be used for the selection of batteries in such a simple way as to determine the quality of the battery as “good” or “defective”. As disclosed in U.S. Pat. No. 6,118,275, for example, the voltage response to current pulse measured for one second with 1 millisecond sampling rate has information about impedance at five different frequencies. Impedance of each frequency has its own unique information about the system. However, only 1 frequency is used for correlation in the prior art, and the other information is lost.
Another problem with these methods lies in the fact that the impedance measurements at a particular frequency are applied to the analysis by mixed state without being separated into capacitance and resistance components and are difficult to use in the fine grouping of the unit cells.
The present invention relating to the selection and grouping of unit cells for a battery pack takes all the frequency regions of the measured impedance spectrum into consideration in the impedance analysis. Also, the present invention fits the impedance spectrum to a physically adequate equivalent circuit model to separate the internal characteristic of the unit cells into resistance and capacitance components and to secure perfect determination of the internal characteristic in connection with the power characteristic of the unit cells. The resistance component as used herein may be approximated to the total DC resistance of the cell corresponding to the resistance during a discharge at a predetermined current.